This invention relates to a system for detecting and imaging objects and structures surrounded by scattering media using light irradiation.
The detection of objects and structures surrounded by a weakly absorbent scattering medium poses a difficult problem in diverse technical fields. For example, traffic technology seeks to achieve sighting of large objects in fog. In medicine, the observation of morphological and anatomical structures in the organs and periphery of the human body is frequently useful. Materials technology has long sought effective methods for detection of flaws and inhomogenities in translucent substances which are not transparent.
For the imaging of objects or structures surrounded by scattering media, transillumination procedures (diaphanoscopy) are customarily employed, in which the object or structure is recognized on the basis of the shadow edges it produces in the scattered light emanating from the opposite side if the medium. Such transillumination processes are applied in medicine, for instance, to determine the course of blood vessels in tissue layers.
Such transillumination procedures are also used to determine the absorptivity of a medium containing scattering or absorbing particles. By measuring the absorption, the concentration of the particles in the medium may be determined. For the state of the art of absorption measurements, see DE-AS No. 2,751,365 and DE-OS No. 2,933,066.
Whether for the detection of objects or the determination of the particle concentration in a scattering medium, the use of a transillumination procedure requires that both sides of the medium be accessible. This is often not the case, however. In the example from the field of medicine in which blood vessels are to be detected in tissue, the tissue layers are often too thick optically to allow the light to pass through. Transillumination procedures are thus inapplicable to such cases.
It is here that the invention can be applied to solve the problem of providing a procedure and apparatus for the detection and imaging of objects and structures surrounded by a scattering medium by means of illumination of and detection from one and the same side of the medium.
The invention is based on recognition of the following facts: if a collimated light beam falls on a scattering medium, at every point along its path inward a portion of the light is absorbed and a portion is scattered. Depending on the size, constitution and density of the scattering centers and the polarization state of the incident light, the scattering can display a variety of predominant behaviors including forward, backward or isotropic characteristics. In the cases discussed here of extremely dense scattering media, which render the buried objects invisible to conventional examination, the scattering can be adequately treated by a model in which the elementary scattering process is isotropic and independent of polarization so that the residual component of the incoming light retains its original polarization. According to this model, the incoming radiation is distributed by multiple scattering inside the medium as follows: the light beam will be considerably broadened inside the medium by virtue of the scattering process. The magnitude of this effect increases with the effective depth of the medium; the residual component of the incoming beam is strongly extinguished due to absorption and scattering of the light in all directions away from the orientation of the incoming beam. Beyond a depth corresponding to the extinction coefficient of the medium for the wavelength of the incoming light beam, the light transport is basically a diffusion process in the radial direction. Assuming that the absorptivity of the medium is low, a large percentage of the incoming light will be returned by virtue of multiple scattering from various depths back to the surface of the medium and be reemitted. This gives rise to a "scattering zone" centered at the position of incidence of the incoming beam and, in the case of visible light, appears as a brightly glowing spot that can have a diameter much greater than that of the incoming beam. In Applied Optics 18, 2286 ff (1979), the distribution of the total power P.sub.o incident on a slab of scattering material into the various modes is plotted as a function of the slab depth. The modes of distribution of the light energy are: P.sub.r the residual beam power reaching the rear wall; P.sub.f the power scattered forward out of the slab; P.sub.b the power scattered backwards in the direction of the beam's source; P.sub.a the power absorbed by the medium. The dependence of the backscattered power on the depth of a sample of a scattering medium can be used to infer the existence of a strongly absorbing region (object or structure) beneath the surface, which has the effect of a local reduction of the thickness of the scattering medium. A global evaluation of the fluctuation of the local depth as a function of the scanning position can, therefore, be utilized for the detection and imaging of objects "buried" in the scattering medium which would not be visible on diffuse illumination of the surface of the medium.